Breath gas analysis is commonly performed to provide information related to a patient's condition. An example of a gas analysis often performed is capnography using an analyzer called a capnograph. Capnography is the monitoring of the time dependent respiratory carbon dioxide (CO2) concentration, which may be used to directly monitor the inhaled and exhaled concentration of CO2, and indirectly monitor the CO2 concentration in a patient's blood. Capnography may provide information about CO2 production, pulmonary (lung) perfusion, alveolar ventilation (alveoli are hollow cavities in the lungs in which gas exchange is being performed) and respiratory patterns. Capnography may also provide information related to a patient's condition during anaesthesia, for example by monitoring the elimination of CO2 from anaesthesia breathing circuit and ventilator. More information regarding capnography may be found in http://www.capnography.com/ and http://www.nda.ox.ac.uk/wfsa/html/u11/u1107_01.htm, which are herein incorporated by reference in its entirety.
In breath analysis systems, for example capnography, breath gas can be sampled either by a mainstream or a sidestream analyzer. In mainstream analyzers the sample chamber is positioned within the patient's gas stream near the patient's end of the breathing system. This arrangement is normally heavier and more cumbersome.
In sidestream analysers gas is drawn from the breathing system by a tube. The tube, which may be connected to an adaptor near the patient's end of the breathing system, delivers the gas to a sampling place (such as a sampling chamber). There are several elements that are generally common to sidestream breath analysis systems (such as capnographs) including, a monitor that continuously samples and monitors the CO2 in a patients breath, airway tube(s) and sampling line(s) which may be flexible tube(s) having narrower diameter(s) than the airway tube(s), and are adapted used to connect between the patient airway tube(s) and the distant analyzer, such as the capnograph monitor. Along this tube, the patient's breath is continuously sampled.
It is usually preferable that the sampling line is clear of liquids in the fluid sample at all times, in order to permit continuous, non-interfered monitoring. Such liquids are common in patient sampling systems, and have several origins, for example:                condensed out liquids from the highly humidified air provided to and exhaled from the patient. These liquids typically accumulate both in the patient airway and in the sampling line tubing;        secretions from the patient, typically found in the patient airway; and        medications or saline solution provided to the patient during Lavage, suction and nebulization procedures.        
Condensed out liquids generally refer to water that condenses out from the humidity (the water vapor in a air or in other gas) in the sampling tubes. Condensed out liquids is a major problem commonly hindering breath analyses, particularly sidestream capnography. The internal humidity levels in the tubes are high especially in proximity to the breath collection area since the exhaled and inhaled breath is humid and relatively warm. This is also the case in intubated patients who are generally artificially ventilated with gas (for example, air) having up to 100% humidity at a temperature normally above ambient temperature (for example, about 34° C.), depending on the airway humidification system and patient needs. The humidity (water vapors) often condenses on the tube particularly as the tube is extended farther from the breath collection area due to the temperature decreases.
Several methods have been developed in order to keep the sampling line free of liquids such as those mentioned above, particularly moisture. Some methods are designed to prevent liquids from entering the sampling line (for example, as described in U.S. Pat. No. 5,857,461) and some are designed to remove such liquids if they entered the sampling line or were created in it.
In addition to the aforementioned preventive steps, it is common practice in side-stream capnography to use tubes that are made of or include drying materials. The internal humidity levels in the tubes, especially in proximity to the breath collection area, are high, and since airway temperature, dictated by the airway humidification system and patient needs, is relatively high, there is a clear need for a material which will bring down the internal humidity of the sampled breath before the humid gas would condense out when flowing towards the gas analyzer (for example capnograph), cooled by the ambient air. One such suitable material is Nafion®.
Nafion® is a copolymer of tetrafluoroethylene (Teflon®) and perfluoro-3,6-dioxa-4-methyl-7-octene-sulfonic acid. Like Teflon, Nafion® is highly resistant to chemical attack, but the presence of its exposed sulfonic acid groups confers unusual properties. Sulfonic acid has a very high water-of-hydration, absorbing 13 molecules of water for every sulfonic acid group in the polymer; consequently, Nafion® absorbs 22% by weight of water. Since the Nafion® specifically reacts with water, gases being dried or processed are usually entirely unaffected. More information regarding Nafion® may be found in http://www.permapure.com/OurTechnology.htm, which is herein incorporated by reference in its entirety. Nafion® tubing, which comprises an ion exchanger, has the ability to equate internal humidity levels with the external ambient humidity.
The efficiency of drying materials such as Nafion® in dehumidifying a gas sample is dependant mainly on the following parameters:                the rate of flow of gas passing through the drying tube (a tube that include a drying material such as Nafion®), wherein the slower the flow rate, the more efficient is the dehumidifying process;        the tube diameter, wherein the smaller the diameter, the more efficient is the dehumidifying process;        the wall thickness, wherein the thinner the wall, the more efficient is the dehumidifying process;        the humidity gradient inside and outside of the tube, wherein the greater the difference in humidity inside and outside, the more efficient is the dehumidifying process;        the temperature of the humid gas, wherein the higher the temperature, the more efficient is the dehumidifying process; and        The movement of air around the drying tube (such as Nafion® tube); wherein the faster the movement, the more efficient is the dehumidifying process.        
Materials like Nafion® tend to be expensive and their cost is dependant on the length of material required. Since gas sampling lines (for example in a capnograph) are disposable in nature, one must design the sample line including the Nafion® tube in the most effective way so as to use the least amount of drying material such as Nafion® necessary for the purpose required. Hence one must use the above listed parameters in order to provide the optimal solution.
Further, the patient airway tubes are typically furnished with large diameters, to address large flows of gas, typically up to about 30 liters per minute. Such patient airway tubes may use heating systems to keep the humidified air from condensing out. On the other hand, gas sampling lines (for example in a capnograph) often use small (internal and external) diameter bore tubings to enable the undisturbed flow of the sampled breath at low flow rates, such as about 50 ml/min. The substantial change in diameters between the patient airway tubes and the sampling tubes may create rapid drop in temperatures when passing from the patient airway, through the sampling port to the sample line. This temperature drop results in rapid condensation of the humidified breath before reaching the drying tube (which may include for example, Nafion®), and hence before the internal humidity can be equated with the lower ambient humidity. Since the temperature generally drops significantly before the gas reaches the drying material (while as aforementioned, drying materials such as Nafion® operate more efficiently at higher temperatures) and since water may have already condensed out in the sampling line before the gas reaches the drying material (while drying materials such as Nafion® are more efficient in removing humidity than water), drying material such as Nafion® are often used in an ineffective manner. Prior art refers to the problem of moisture in gas sampling tubes. U.S. Pat. No. 6,783,573, for example, refers to the “moisture problem” and teaches away from placing a dryer mechanism in the connector adjacent to, or proximate to, the respiratory output device (e.g., the mask, cannula, or etc.). U.S. Pat. No. 6,783,573 specifically states that this approach has drawbacks. Moreover, U.S. Pat. No. 6,783,573 states “the dryer mechanism disposed proximate the patient is ineffective, . . . ” (column 1, lines 64-65). U.S. Pat. No. 6,783,573 is incorporated herein by reference in its entirety.
There is thus a need for sampling systems that are adapted to reduce the amount of liquids, particularly moisture, that form along the sampling tubes.